Emotion is complex, and the term has no single universally accepted definition. The study of emotions is part of psychology, neuroscience, and, more recently, artificial intelligence.
According to Sloman, emotions are cognitive processes. Some authors emphasize the difference between human emotions and the affective behavior of animals.
In Paul D. MacLean's classic Triune brain model, emotions are defined as the responses of the Mammalian cortex. Emotion competes with even more instinctive responses from the Reptilian cortex and the more logical, reasoning neocortex. However, current research on the neural circuitry of emotion suggests that emotion is an essential part of human decision-making and planning, and that the famous distinction made by Descartes between reason and emotion is not as clear as it seems.
We often talk about brains as information-processing systems, but any account of the brain that lacks an account of emotions, motivations, fears, and hopes is incomplete. Emotions are measurable physical responses to salient stimuli: the increased heartbeat and perspiration that accompany fear, the freezing response of a rat in the presence of a cat, or the extra muscle tension that accompanies anger. Feelings, on the other hand, are the subjective experiences that sometimes accompany these processes: the sensations of happiness, envy, sadness, and so on. Emotions seem to employ largely unconscious machinery—for example, brain areas involved in emotion will respond to angry faces that are briefly presented and then rapidly masked, even when subjects are unaware of having seen the face. Across cultures the expression of basic emotions is remarkably similar, and as Darwin observed, it is also similar across all mammals. There are even strong similarities in physiological responses among humans, reptiles, and birds when showing fear, anger, or parental love.
Modern views propose that emotions are brain states that quickly assign value to outcomes and provide a simple plan of action. Thus, emotion can be viewed as a type of computation, a rapid, automatic summary that initiates appropriate actions. When a bear is galloping toward you, the rising fear directs your brain to do the right things (determining an escape route) instead of all the other things it could be doing (rounding out your grocery list). When it comes to perception, you can spot an object more quickly if it is, say, a spider rather than a roll of tape. In the realm of memory, emotional events are laid down differently by a parallel memory system involving a brain area called the amygdala.
One goal of emotional neuroscience is to understand the nature of the many disorders of emotion, depression being the most common and costly. Impulsive aggression and violence are also thought to be consequences of faulty emotion regulation.